Evaluation of Selected Processes Controlling Nitrogen Loss in a Flooded Soil

The roles of NH.-N diffusion, NH.-N oxidation, NO3-N diffusion, and NO3-N reduction in controlling N loss from continuously flooded soil were evaluated in independent experiments. Applied NH,-N moved from zones of high NH4-N concentration to the zones of low NH4-N concentration. The average diffusion coefficient (D) for NH.-N in flooded soil ranged from 0.059 to 0.216 cm" day for different soils. Diffusion coefficients were influenced by soil type and soil-water content. Rate of NH4-N oxidation in the aerobic layer of flooded soil range from 1.2 to 3.5 /tg g' day in different soils. Nitrate diffusion into the anaerobic soil layer ranged from 0.96 to 1.91 cm day, whereas NO3-N reduction rates were 0.32 to 0.52 day. The slow rate of NH4-N diffusion from the anaerobic soil layer to the aerobic soil layer and the slow rate of NHt-N oxidation in the aerobic soil layer indicate that these two processes are limiting steps in controlling N loss. Nitrate diffusion into the anaerobic soil layer and NO3-N reduction in the anaerobic soil layer were found to proceed at a faster rate and are not likely to limit N loss from flooded soil. Additional Index Words: Ammonium diffusion, ammonium oxidation, nitrate diffusion, nitrate reduction, floodwater, waterlogged soils. Reddy, K. R., W. H. Patrick, Jr., and R. E. Phillips. 1980. Evaluation of selected processes controlling nitrogen loss in a flooded soil. Soil Sci. Soc. Am. J. 44:1241-1246. N ITROGEN REACTIONS in flooded soil are greatly influenced by the presence of O2 in the atmosphere overlying floodwater. This O2 moves through the floodwater and reaches the soil surface, thereby causing the floodwater and the soil surface to be oxidized. The thickness of the aerobic soil layer (also called oxidized or nitrification zone) depends on the rate of O2 movement through the floodwater and soil and the rate of O2 consumption by the soil. Generally, the thick-, ness of this layer varies from a few mm in soils of high biological activity to 1 to 2 cm in soils of low biological activity. Underlying the aerobic soil layer is the anaerobic soil layer (also called reduced or denitrification zone), which is devoid of O2. The major inorganic form of N in flooded soil is NH4-N. Ammonium N present in the surface aerobic soil layer can be readily oxidized to NO3-N. The NO3N thus formed during the nitrification moves down into the anaerobic soil layer and undergoes denitrification. Several research workers, namely Pearsall (1950), Mitsui (1954), Patnaik (1965), Patrick and Tusneem (1972), Patrick and Delaune (1972), and 1 Joint contribution from the Lab. of Flooded Soils and Sediments, Agronomy Dep., Louisiana State Univ., Baton Rouge, La., Dep. of Agronomy, Univ. of Kentucky, Lexington. Ky., and Univ. of Florida, Agric. Res. & Educ. Center, Sanford, Fla. The research was conducted while the senior author was at Louisiana State Univ., Baton Rouge, La. Received 24 Mar. 1980. Approved 18 June 1980. 2 Assistant Professor, Agric. Res. & Educ. Center, Univ. of Florida, Sanford, FL 32771. "Boyd Professor, Center for Wetland Resources, Louisiana State Univ., Baton Rouge, LA 70803. 'Professor, Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40506. Chen et al. (1972), have reported this mechanism of N loss in flooded soils and sediments. More recently, Reddy et al. (1976) and Patrick and Reddy (1976) have demonstrated N loss due to the sequential process of NH4-N diffusion from the anaerobic soil layer to the overlying aerobic soil layer, oxidation of NH4-N in the aerobic soil layer, NO3-N diffusion from the aerobic soil layer to the anaerobic soil layer, followed by the reduction of NO3-N to gaseous end products. Although the rate of each of these processes in determining N loss was identified in an indirect way, their relative rates were not determined quantitatively. The objectives of the present investigation were to evaluate (i) the rate of NH4-N diffusion from the anaerobic layer to the aerobic layer, (ii) the rate of NH4-N oxidation in the aerobic soil layer, (iii) the rate of NO3-N diffusion down to the anaerobic layer, and (iv) the rate of NO3-N reduction in the anaerobic soil layer. Schematic presentation of these processes are shown in Fig. 1. MATERIALS AND METHODS The soils used in the present study were Crowley silty loam (fine, montmorillonitic, thermic Typic Albaqualfs); Midland silty clay loam (fine, montmorillonitic, thermic Vertic Ochraqualfs) obtained from the Rice Experiment Station, Crowley, La; Mhoon silty clay loam (fine-silty, mixed, nonacid, thermic Typic Fluvaquents); and Commerce silty clay loam (fine-silty, mixed, nonacid, thermic Aerie Fluvaquents) obtained from the Mississippi River flood-plain near Baton Rouge, La. Gallion sandy loam (tine-silly, mixed, thermic Typic Hapludalfs) was obtained from the Ouachita River flood-plain near Monroe, La. The soils were air-dried, ground to pass through a 10-mesh sieve, mixed thoroughly, and stored in a tightly sealed container. For each independent study, a subsample of the soil was taken and ground to pass through a 40-mesh sieve. Selected physical and chemical properties of these soils are shown in Table 1. The N source used in the experiments was either ammonium sulfate or potassium nitrate, which was thoroughly mixed with the soil. This was accomplished by mixing the N source with a small amount of soil in a porcelain mortar and then mixing this with a larger amount of soil to obtain the desired concentration of NH.-N or NCyN. Later, the soil containing the N